Smelter gases normally contain significant quantities of contaminants which must be removed prior to discharge of the gases to the atmosphere or prior to feeding the gases to sulfuric acid or sulfur dioxide manufacturing facilities.
The gas contaminants may include unreacted solids or concentrate, calcined materials, slag, other feed materials, metals which are more volatile than the metal being recovered, and a variety of other elements such as mercury, arsenic, antimony, bismuth, selenium, tellurium, and sulfur. At furnace temperatures these impurities may be solid, liquid, or gaseous in form. As the gas cools the molten materials solidify, and some of the gaseous impurities start to form solid or liquid particles or droplets which often are found as fine fumes having particle sizes well below one micron. These fine particulates are almost impossible to remove by inertial separation or scrubbing means and require the use of expensive wet electrostatic precipitators.
Smelter gases are generated at a variety of temperatures depending on the metal being recovered. Typical temperatures for zinc are in the range 900 to 1,000 degrees centigrade while copper temperatures are closer to 1,200 degrees centigrade. In classical smelter gas cleaning practice, the hot gases from the smelting furnace are cooled initially to temperatures in the range 300 to 350 degrees centigrade using boilers in which high pressure steam is generated. The boilers are usually of the water-in-tube type and are designed to handle high dust loads. From the boilers the gas then flows to hot electrostatic precipitators where dust is removed. Typical dust removal efficiencies may run as high as 99%, but in many cases lower efficiencies have had to be accepted. Various considerations are taken into account in selecting an operating temperature range for the precipitator including the temperature which can be tolerated by carbon steel, which is typically used in hot precipitator construction, the dew point temperature of the gas which dictates the lowest acceptable metal temperature in the precipitator, and the temperature which gives the best electrical conductivity of the particles being collected. Accordingly, the resultant operating temperature is a compromise.
In some cases, the smelter furnace off-gas is quenched, as opposed to being cooled in a boiler, the quenching serving to reduce the temperature to an extent suitable for hot precipitators. In relatively few cases, where oxygen flash smelting is used, the gas is cooled down by quenching to the wet-bulb temperature and hot electrostatic precipitators and boilers are totally avoided. In such cases, the wet gas cleaning systems have to handle the complete flow of solids and other contaminants from the furnace. These variations are the exception rather than the rule, and for the purpose of the present discussion classic practice has been assumed.
From the electrostatic precipitators the gas next flows to the wet scrubbing system. The first stage in this system saturates the gas with water by contacting the gas with either water or a weak sulfuric acid solution. Depending on the inlet gas composition and temperature and the concentration of the acid used to saturate the gas, the temperature of the saturated gas will normally lie in the range of 55 to 85 degrees centigrade. The water vapour content in this gas can range as high as 40%. This first quenching operation does some cleaning but is primarily designed to cool the gas through the temperature range in which the gas is most corrosive, to saturation temperature. In the first quenching step the bulk of the gaseous impurities condense out in a variety of forms which offer different difficulties of removal. Ordinary solid particles and droplets are relatively easy to remove by inertial separation means. Fine chemical fumes, which can be formed by such substances such as H.sub.2 SO.sub.4, As.sub.2 O.sub.3, Pb, and SeO.sub.2, are normally much smaller (in the range 0.1 to 1 micron) and can only be removed by high energy scrubbing or by electrostatic precipitators. Mercury and the water vapour which is present in excess of that required to form H.sub.2 SO.sub.4 cannot be removed at this stage. The gas must be cooled to lower temperatures in the range 30 to 40 degrees centigrade for mercury removal.
The saturated gas now passes to wet scrubbers and gas coolers where the bulk of the impurities are removed and the water vapour content is reduced to a level consistent with the quality of acid being produced. This equipment can include packed towers, plate type scrubbers, gas coolers, indirect gas coolers where the gas is contacted with cooled weak acid, venturi scrubbers and a wide variety of other devices. Typically the gas cleaning system will consist of a train of such devices.
From these devices, the gas which now has been cleaned of almost all the solid particles and contains primarily acid mist, passes to a series of wet electrostatic precipitators where the remaining contaminants are removed.
Wet electrostatic precipitators are predominantly of two types, one type comprising wire electrodes located in tubes and other comprising wire electrodes located between collecting plates. In both cases, a corona discharge is maintained on the wire and the particles are charged by electrical charges generated in the corona. The charged particles then move, under the influence of the electrical field between the wire and the grounded surface, to the collection surface. A typical electrostatic precipitator will have a gas residence time in the range of 4 seconds and an efficiency of 98%. Where tube type units are used, the tubes are typically 250 mm in diameter and tube lengths run to as much as 6 meters. Frequently, where larger residence times and efficiencies are required, such units are arranged in series and a large plant can require as many as 10 or 12 of such units.
Materials used to construct such devices include carbon-steel, lead, lead-lined steel, stainless steels, and a variety of plastics.
A variety of type of precipitators are available, including round tube, hexagonal tube, square tube, concentric ring, and plate type units.
Removal of the process heat from the gas, as discussed above, is normally carried out in stages. The first stage is in the boilers, after the furnace, where the gas is cooled to 300-350 degrees centigrade. In the next step, the gas is adiabatically saturated by contacting the hot inlet gas with a recirculating stream of scrubbing acid. The recirculating stream may have a composition ranging from 1 to 50% acid. It includes impurities previously removed from the gas. While the gas temperature drops drastically in this stage, the drop in temperature is compensated for by a significant increase in the water vapour content. Next, the gas flows to a heat removal stage in which direct and/or indirect contact coolers are employed. These units operate by a variety of means including simple condensation cooling in heat transfer apparatus, direct contact with indirectly cooled weak acid streams in such devices as venturi scrubbers, plate scrubbers, or packed towers and combinations of the above. The complexity and the expense of the heat and excess moisture removal can often pose a more serious problem to the designer of metallurgical plants than the basic gas cleaning problems described above. Depending on the approach used, this cooling may involve anywhere from one to three heat removal steps. A plate scrubber, for example, contacts the gas in two different sections which are in series with respect to the flow of gas. The upper section, containing plates, is cooled, while the lower first section, using sprays, is frequently uncooled. In such scrubbers, the heat is transferred from the gas to the countercurrent streams of weak acid which themselves are cooled by cooling water in separate exchangers. Where condensers are used, anywhere from one to three units may be found in series depending on the type of condenser used, and the cooling water used may flow from one unit to the next or in parallel through all units.
Many reasons exist for better gas cleaning systems. The capital cost of electrostatic precipitator units is high, good gas distribution between and within such units is difficult to ensure, and the units occupy a large amount of space. Also, the performance of such units is usually not good enough for most downstream plants. The development of alternatives has covered the whole range of the cleaning problem from the saturation devices to the precipitators. However, most prior art patents relevant to this field are directed to improvements of single stages of the gas cleaning apparatus and little effort appears to have been spent on integrated approaches which take advantage of all of the equipment required for smelter gas treatment duty. The background art in the field of gas cleaning is extremely large as there are many fields where gases must be cleaned. Notwithstanding this prior art, an integrated approach to cleaning of gases, where one has such mixtures as one finds in the off-gases from metallurgical furnaces, has not been considered and there is no general agreement on how such cleaning should be done. Prior art patents which address the objects of the present invention by making more effective use of the electrostatic principle, are even more limited in number.
One such prior art patent is U.S. Pat. No. 3,874,858. This patent covers a process comprising a gas particle charging step followed by passage of the charged gas through an irrigated packed bed or fiber bed where the particles are attracted to the uncharged packing or fibers. The approach does not use an imposed electrostatic field to cause the particles to flow to the neutral surface but counts on relatively low velocities and significant residence time to permit the charged particles to migrate to the uncharged surface. U.S. Pat. No. 3,958,958 contains the relevant apparatus claims associated with U.S. Pat. No. 3,874,858.
U.S Pat. No. 4,778,493 also discusses electrostatic precipitation and describes techniques by which the charge on the smaller particles can be drastically increased. In this patent, particulate contaminants are pulse charged in different regions of the gas cleaning process, starting with the charging of the fine particles in the absence of a static electrical field. More conventional charging in the presence of an electrostatic field then follows with the field causing the particles to move to the collecting surface. After this collection, electric bombardment is used to charge the few remaining particles. Collection in this case is again by motion in an electrostatic field as in the standard electrostatic precipitator.